Extending 4×4 Deck Posts

Around here, 4x4s are not uncommon. This table shows that a 4x4 pine post that is 6 ft or shorter can support a load area of up to 10 ft² assuming 50 lbs/ft².

Don't trust me or a random table on the internet; make our own calculations from tables in the building code or other trusted sources and consider having them checked by an engineer.

Typical wood deck railings are 4x4 posts, 2x4 rails, 2x2 balusters and a 2x6 railing cap on top. Wood railing is chunky but it needs to be to prevent people falling through it in the event of a accident. It is not just leaning that is the concern but 1 or 2 200+ lb people failing into the weakest spot (middle of the rail) that it needs to handle. But when it is done right, stained a nice color, sealed and has quality work, it does look very nice. You can upgrade the look by having post caps or switching out the wood balusters for metal ones for not much more.

If you want thinner posts you need to switch materials, like metal, but that will probably double your costs. With metal posts, come metal rails which then require metal balusters. All this metal requires a metal worker to fabricate offsite and then bring to site for assembly and install.